Barrier membranes useful for containing fluids, including liquids and/or gases, in a controlled manner, have been employed for years in a wide variety of different products ranging from bladders useful in inflatable objects, including vehicle tires and sporting goods for example; to accumulators used on heavy machinery; to cushioning devices useful in footwear. Regardless of the intended use, desirable barrier membranes must generally be flexible, resistant to environmental degradation and exhibit excellent gas transmission controls. Often, however, materials which exhibit acceptable flexibility characteristics tend to have an unacceptably low level of resistance to gas permeation. In contrast, materials which exhibit an acceptable level of resistance to gas permeation tend to have an unacceptably low level of flexibility.
In an attempt to address the concerns of both flexibility and imperviousness to gases, U.S. Pat. No. 5,036,110 which issued Jun. 30, 1991, to Moreaux describes resilient membranes for fitting hydropneumatic accumulators. According to Moreaux '110, the membrane disclosed consists of a film formed from a graft polymer which is the reaction product of an aromatic thermoplastic polyurethane with a copolymer of ethylene and vinyl alcohol, with this film being sandwiched between layers of thermoplastic polyurethane to form a laminate. While Moreaux '110 attempts to address the concerns in the art relating to flexibility and imperviousness to gases, a perceived drawback of Moreaux is that the film described is not processable utilizing conventional sheet extrusion techniques. Thus, the present invention is directed to barrier membranes which are flexible, have good resistance to gas transmission, and under certain embodiments are processable into laminates utilizing conventional sheet extrusion techniques which are highly resistant to delamination.
While it should be understood by those skilled in the art upon review of the following specification and claims that the barrier membranes of the present invention have a broad range of applications, including but not limited to bladders for inflatable objects such as footballs, basketballs, soccer balls and inner tubes; films for food packaging; as well as the production of fuel lines and fuel storage tanks to name a few, still other applications are possible. For example, one highly desirable application for the barrier membrane of the present invention include their use in forming accumulators which are operable under high pressure environments such as hydraulic accumulators as will be discussed in greater detail below.
For convenience, but without limitation, the barrier membranes of the present invention will hereinafter be described generally in terms of either accumulators or in terms of still another highly desirable application, namely for cushioning devices used in footwear. In order to fully discuss the applicability of the barrier membranes in terms of cushioning devices for footwear, a description of footwear in general is believed to be necessary.
Footwear, or more precisely, shoes generally include two major categories of components namely, a shoe upper and the sole. The general purpose of the shoe upper is to snugly and comfortably enclose the foot. Ideally, the shoe upper should be made from an attractive, highly durable, yet comfortable material or combination of materials. The sole, which also can be made from one or more durable materials, is particularly designed to provide traction, protect the wearer's feet and body during use which Is consistent with the design of the shoe. The considerable forces generated dunng athletic activities require that the sole of an athletic shoe provide enhanced protection and shock absorption for the feet, ankles and legs of the wearer. For example, impacts which occur during running activities can generate forces of up to 2-3 times the body weight of an individual while certain other activities such as, for example, playing basketball have been known to generate forces of up to approximately 6-10 times an individual's body weight. Accordingly, many shoes and, more particularly, many athletic shoes are now provided with some type of resilient, shock-absorbent material or shock-absorbent components to cushion the user during strenuous athletic activity. Such resilient, shock-absorbent materials or components have now commonly come to be referred to in the shoe manufacturing industry as the mid-sole.
It has therefore been a focus of the industry to seek mid-sole designs which achieve an effective impact response in which both adequate shock absorption and resiliency are appropriately taken into account. Such resilient, shock-absorbent materials or components could also be applied to the insole portion of the shoe, which is generally defined as the portion of the shoe upper directly underlining the plantar surface of the foot.
A particular focus in the shoe manufacturing industry has been to seek mid-sole or insert structure designs which are adapted to contain fluids, in either the liquid or gaseous state, or both. Examples of gas-filled structures which are utilized within the soles of shoes are shown in U.S. Pat. No. 900,867 entitled "Cushion for Footwear" which issued Oct. 13, 1908, to Miller; U.S. Pat. No. 1,069,001 entitled "Cushioned Sole and Heel for Shoes" which issued Jul. 29, 1913, to Guy; U.S. Pat. No. 1,304,915 entitled "Pneumatic Insole" which issued May 27, 1919, to Spinney; U.S. Pat. No. 1,514,468 entitled "Arch Cushion" which issued Nov. 4, 1924, to Schopf; U.S. Pat. No. 2,080,469 entitled "Pneumatic Foot Support" which issued May 18, 1937, to Gilbert; U.S. Pat. No. 2,645,865 entitled "Cushioning Insole for Shoes" which issued Jul. 21, 1953, to Towne; U.S. Pat. No. 2,677,906 entitled "Cushioned Inner Sole for Shoes and Method of Making the Same" which issued May 11, 1954, to Reed; U.S. Pat. No. 4,183,156 entitled "Insole Construction for Articles of Footwear" which issued Jan. 15, 1980, to Rudy; U.S. Pat. No. 4,219,945 entitled "Footwear" which issued Sep. 2, 1980, also to Rudy; U.S. Pat. No. 4,722,131 entitled "Air Cushion Shoe Sole" which issued Feb. 2, 1988, to Huang; and U.S. Pat. No. 4,864,738 entitled "Sole Construction for Footwear" which issued Sep. 12, 1989, to Horovitz. As will be recognized by those skilled in the art, such gas filled structures often referred to in the shoe manufacturing industry as "bladders" typically fall into two broad categories, namely (1) "permanently" inflated systems such as those disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 and (2) pump and valve adjustable systems as exemplified by U.S. Pat. No. 4,722,131. By way of further example, athletic shoes of the type disclosed in U.S. Pat. No. 4,182,156 which include "permanently" inflated bladders have been successfully sold under the trade mark "AIR SOLE" and other trademarks by Nike, Inc. of Beaverton, Oreg. To date, millions of pairs of athletic shoes of this type have been sold in the United States and throughout the world.
The permanently inflated bladders are typically constructed under methods using a flexible thermoplastic material which is inflated with a large molecule, low solubility coefficient gas otherwise referred to in the industry as a "super gas," such as SF.sub.6. By way of example, U.S. Pat. No. 4,340,626 entitled "Diffusion Pumping Apparatus Self-inflating Device" which issued Jul. 20, 1982, to Rudy, which is expressly incorporated herein by reference, discloses a pair of elastomeric, selectively permeable sheets of film which are formed into a bladder and thereafter inflated with a gas or mixture of gases to a prescribed pressure which preferably is above atmospheric pressure. The gas or gases utilized ideally have a relatively low diffusion rate through the selectively permeable bladder to the exterior environment while gases such as nitrogen, oxygen and argon which are contained in the atmosphere and have a relatively high diffusion rate are able to penetrate the bladder. This produces an increase in the total pressure within the bladder, by the addition of the partial pressures of the nitrogen, oxygen and argon from the atmosphere to the partial pressures of the gas or gases contained initially injected into the bladder upon inflation. This concept of a relative one-way addition of gases to enhance the total pressure of the bladder is now known as "diffusion pumping."
Under the diffusion pumping system and depending upon the bladder material used and the choice of gas or gases contained therein, there is a period of time involved before a steady state of internal pressure is achieved. For example, oxygen tends to diffuse into the bladder rather quickly with the effect being an increase in the internal pressure of approximately 2.5 psi. In contrast, over the course of a number of weeks nitrogen gas will gradually diffuse into the bladder resulting in an increase of pressure to approximately 12.0 psi The gradual increase in bladder pressure typically causes an increase in tension in the bladder skin, resulting in a volume increase due to stretching. This effect is commonly referred to in the industry as "tensile relaxation" or "creep." Thus, it is of significant importance which materials are chosen for the bladder and the choice of the captive gas mixture utilized to initially inflate the bladder to achieve a bladder which is essentially permanently inflated at a desired internal pressure and which maintains a desired internal pressure over an extended period of time.
With regard to the systems utilized within the shoe manufacturing industry prior to and shortly after the introduction of the AIR SOLE.TM. athletic shoes, many of the mid-sole bladders consisted of a single layer gas barrier type films made from polyvinylidene chloride based materials such as SARAN.TM. (which is a registered trademark of the Dow Chemical Co.) and which by their nature are rigid plastics, having relatively poor flex fatigue, heat sealability and elasticity. Still further, bladder films made under techniques such as laminations and coatings which involve one or more barrier materials in combination with a flexible bladder material (such as various thermoplastics) can potentially present a wide variety of problems to solve. Such difficulties with composite constructions include layer separation, peeling, gas diffusion or capillary action at weld interfaces, low elongation which leads to wrinkling of the inflated product, cloudy appearing finished bladders, reduced puncture resistance and tear strength, resistance to formation via blow-molding and/or heat-sealing and R-F welding, high cost processing, and difficulty with foam encapsulation and adhesive bonding, among others.
Yet another issue with previously known bladders is the use of tie-layers or adhesives in preparing laminates. The use of such tie layers or adhesives generally prevent regrinding and recycling of any waste materials created during product formation back into an usable product, and thus, also contribute to high cost of manufacturing and relative waste. These and other short comings of the prior art are described in more extensive detail in U.S. Pat. Nos. 4,340,626; 4,936,029 and 5,042,176, all of which are hereby expressly incorporated by reference.
With the extensive commercial success of the products such as the AIR SOLE.TM. shoes, consumers have been able to enjoy a product with a long service life, superior shock absorbency and resiliency, reasonable cost, and inflation stability, without having to resort to pumps and valves. Thus, in light of the significant commercial acceptance and success that has been achieved through the use of long life inflated gas filled bladders, it is highly desirable to develop advancements relating to such products. The goal then is to provide flexible, "permanently" inflated, gas-filled shoe cushioning components which meet, and hopefully exceed, performance achieved by such products as the AIR SOLE.TM. athletic shoes offered by Nike, Inc.
One key area of potential advancement stems from a recognition that captive gases other than the large molecule, low solubility coefficient "super gases" as described in the '156, '945 and '738 patents utilized can be replaced with less costly and possibly more environmentally friendly gases. For example, U.S. Pat. Nos. 4,936,029 and 5,042,176 specifically discuss the methods of producing a flexible bladder film that essentially maintains permanent inflation through the use of nitrogen as the captive gas. As further described in U.S. Pat. No. 4,906,502, also specifically incorporated herein by reference, many of the perceived problems discussed in the '029 and '176 patents are solved by the incorporation of mechanical barriers of crystalline material into the flexible film such as fabrics, filaments, scrims and meshes. Again, significant commercial success for footwear products using the technology described in '502 patent under the trademark TENSILE AIR.TM. sold by Nike, Inc. has been achieved. The bladders utilized therein are typically comprised of a thermoplastic urethane laminated to a core fabric three-dimensional, double bar Raschel knit nylon fabric, having SF.sub.6 as the captive gas contained therein.
By way of example, an accepted method of measuring the relative permeance, permeability and diffusion of different film materials is set forth in the procedure designated as ASTM D-1434-82. According to ASTM D-1434-82, permeance, permeability and diffusion are measured by the following formulas: ##EQU1##
By utilizing the above listed formulas, the gas transmission rate in combination with a constant pressure differential and the film's thickness, can be utilized to define the movement of gas under specific conditions. In this regard, the preferred gas transmission rate (GTR) for a bladder in an athletic shoe component which seeks to meet the rigorous demands of fatigue resistance imposed by heavy and repeated impacts has a gas transmission rate (GTR) value of approximately 10.0 or lower and, even more preferably, a (GTR) value of 2.0 or lower, for bladders having an average thickness of approximately 20 mils.
In addition to the aforementioned, the '029 and '176 patents also discuss problems encountered with previous attempts to use co-laminated combinations of plastic material which operate as barriers to oxygen. In this regard, the principal concern was the lack of fatigue resistance of the barrier layer. As described in the '176 patent, a satisfactory co-lamination of polyvinylidene chloride (such as SARAN.RTM.) and a urethane elastomer would require an intermediate bonding agent. Under such a construction, relatively complicated and expensive processing controls such as strict time-temperature relationships and the use of heated platens and pressures, coupled with a cold press to freeze the materials together under pressure would be required. Additionally, using adhesive tie layers or incorporating crystalline components into the flexible film at high enough levels to accomplish a gas transmission rate of 10 or less, reduces the flexibility of the film.
Cushioning devices which specifically eliminate adhesive tie layers have been known to separate or de-laminate especially along seams and edges. Thus it has been a relatively recent focus of the industry to develop cushioning devices which reduce or eliminate the occurrence of delamination ideally without the use of a "tie layer." In this regard, the cushioning devices disclosed in co-pending U.S. application patent Ser. Nos. 08/299,286 and 08/299,287 eliminate adhesives tie layers by providing membranes including a first layer of thermoplastic urethane and a second layer including a copolymer of ethylene and vinyl alcohol wherein hydrogen bonding occurs over a segment of the membranes between the first and second layers. While the cushioning devices disclosed in U.S. patent application Ser. No. 08/299,287 and the laminated flexible barrier membranes of U.S. patent application Ser. No. 08/299,286 are believed to offer a significant improvement in the art, still further improvements are offered according to the teachings of the present invention.
It is therefore, a principal object of the present invention to provide barrier membranes which offer enhanced flexibility, durability and resistance to the undesired transmission of fluids therethrough.
It is another object of the present invention to provide barrier membranes which can essentially be permanently inflated with nitrogen or another environmentally desirable gas or combination of gases wherein the barrier membrane provides for a gas transmission rate value of 10.0 or less, based on a 20 mils average thickness.
It is still another object of the present invention to provide barrier membranes and, particularly those employed as cushioning devices with improved clarity and consistency.
It is yet another object of the present invention to provide barrier membranes which can be formed into laminated objects such as cushioning devices or accumulators which resist delamination and do not require a tie layer between the barrier layer and the flexible layers
It is yet another object of the present invention to provide barrier layers which are reprocessable.
It is a further object of the present invention to provide barrier membranes which are formable utilizing the various techniques including, but not limited to, blow-molding, tubing, sheet extrusion, vacuum-forming, heat-sealing and RF welding.
Still another object of the present invention is to provide barrier membranes which prevent gas from escaping along interfaces between the layers in laminated embodiments and particularly along seems via capillary action.
It is yet another object of the present invention to provide a barrier membrane which allows for normal footwear processing such as encapsulation within a formable material.
While the aforementioned objects provide guidance as to possible applications for the barrier membranes of the present invention, it should be recognized by those skilled in the art that the recited objects are not intended to be exhaustive or limiting.